The purpose of the present study were (1) to investigate the Tweed triangle in adult Saudi male and female; (2) to investigate the correlation between the angulation of the lower incisor to the mandibular plane and facial soft tissue lines; and (3) to compare the findings of the adult Saudi to the established Caucasian data. The lateral skull radiographs of 60 adults Saudi (30 male and 30 female) with normal occlusion and balance profile were selected from the 185 lateral cephalometric x-rays of the dental students, King Saud University, College of Dentistry. The age range was from 24 to 28 years. The lateral cephalometric x-ray were traced and digitized. From the digitized data, the ANB angle, Tweed triangle and the angulation fo the lower incisor to the Apog and NB lines were obtained. For the soft tissue assessment the Z-angle, the H-angle, the nasiolabial angle and the lower lip to the esthetic line were measured.
The results of the comparisons for the Tweed triangle and the other variables indicated that no significant differences were observed between the adult Saudi male and female except for the angulation of the lower incisor to the mandibular plane and the Apog line respectively. Therefore, the data for the adult Saudi male and female were pooled together.
To investigate the claim that "there is a definite correlation between balanced facial lines and the position of the mandibular incisors with the relation to basal bone" the lower incisor angulation to the mandibular plane was correlated to the other variables. The values of the coefficient correlation between the lower incisor angulation to the mandibular plane and the facial soft tissue lines were very low and not significant. Only three variables showed significant correlation, however, these variables were related to the long axis of the lower incisor and not to the facial soft tissue. Therefore the above claim was not supported.
No significant differences were found between the pooled adult Saudi and the Caucasian except for the FMA. The mean value of the adult Saudi was smaller than the Caucasian.